You are effectively asking two questions here:
1/ How long will it take to see improvements in health and thus reduced absenteeism and improved productivity?
2/ How long will it take for this to lead to increased staff retention?
Lets look at the first question because the research answers that for us.
It really depends on the initial health status of individual participants. We generally measure an individual's health status using the number of health risk factors they have. Both Australian and US research has indicated this is ~2.7 - 2.9 on average. While most risk factors are modifiable through changes in lifestyle, some (such as age, gender and family history) are not.
For an individual suffering from 0-1 health risk factors then it is unlikely we will see much of an improvement in health status, and if we do it will be minor.
For those suffering 3-4 risk factors, and even more so those above this group, the benefits of a wellness program will be more substantial, both in terms of the magnitude of improvement and the rate of improvement.
Health and fitness are 'chronic' states and long term change is not a result of acute changes. Research has indicated that improvements in the health of an organisation's staff (and therefore associated financial benefits) can be improved by 50-60%, however you are only looking at about a 10% improvement per year.
Please note however that you will not get everyone involved in a voluntary wellness program, and for those involved in the program, improvements will vary greatly.
Question 2 is much more complicated as you are trying to relate this these potential improvements to staff retention which is an indirect benefit of a wellness program. Staff generally value both the short term and long term implementation of a wellness program, which indicates that short term benefits may exist in this area. However, to accurately determine exactly how this correlates to staff retention is a research project in itself.
Hope this help.
Ben
Ben Wisbey
Managing Director
FitSense Australia